1. Field of Invention
The present invention relates to a method of and an apparatus for controlling combustion of an engine, and more particularly to a method of and an apparatus for controlling combustion of an engine to improve unstable ignition and knocking which may be generated in an engine to which a diesel-gasoline mixed combustion mode and a gasoline mixed combustion mode are applied to low load and high load driving zones.
2. Description of Related Art
Today, two types of engines are widely used, one is a gasoline engine of a spark ignition (SI) type and the other is a diesel engine of a compression ignition (CI) type.
The SI type engine may be classified into an indirect injection type to inject fuel into a throttle body or an intake manifold and to intake the fuel with air at the intake stroke and a gasoline direct injection (GDI) type to inject fuel directly into a combustion chamber to burn for the improvement of engine performance and fuel ratio and the reduction of exhaust gas.
A port fuel injection (PFI) type engine, as an example of the indirection injection type engine, employs a premixed combustion in which a combustible mixture consisting of the fuel injected into the intake port of the intake manifold and air is introduced into the combustion chamber and is ignited and burnt by an ignition plug in a uniform mixture.
The GDI engine injects the gasoline fuel directly into the combustion chamber similar to in the CI diesel engine and thus lean burn is enabled and advantageous in view of engine performance, fuel ratio, and exhaust gas in comparison to the indirect injection type engine.
In general, the SI gasoline engine has a limit to increase a compression ratio (CR) because the higher risk knocking may occur the higher the CR is, and has poor thermal efficiency and fuel ratio than those of the CI engine.
In order to comply with the gradually strengthening regulations on CO2 and meet the customer demands, it is necessary to increase the CR of an engine but it needs also a technology of preventing knocking from being generated at the increased CR.
In a high CR gasoline engine, knocking is restrained and pumping loss is reduced when an exhaust gas recirculation (EGR) device is applied to restrain the knocking, but the existing ignition plug cannot obtain good ignition or makes NOx increase.
Meanwhile, combustion property of the diesel engine is to employ the compression ignition using self-ignition property of diesel fuel and uses high compression ratio to make air in the combustion chamber high pressure and high temperature state, so that the diesel engine has superior thermal efficiency and fuel ratio than the gasoline engine.
In recent, as unexceptional climate changes such as global warming are reported all around the world, environmental regulations affect in many countries, especially advanced countries, more particularly in automobile industrial.
In order to meet the environmental regulation, many researches and developments for the compression ignition engine are being conducted due to the advantages such as a small quantity of unburned hydrocarbon (UHC) and CO2. However, the CI engine discharges a great deal of NOx and particulate matters (PM) caused by a not-uniform air-fuel mixture and needs an expensive after-treatment device such as a diesel particulate filter (DPF) and a DeNOx for reduction of NOx and PM to meet the regulations.
To overcome the above-mentioned disadvantages, active researches and developments for a homogeneous charge compression ignition (HCCI) of making the mixture in the combustion chamber homogeneous to remove an area where a fuel is concentrated known as a PM generating area and of reduce drastic heat are carried out.
The HCCI is advantageous to improve fuel ratio through the compression ignition and lean burn in comparison to the gasoline engine and to reduce PM/NOx through the premixed combustion in comparison to the diesel engine.
However, the HCCI is disadvantageous in difficult control of ignition and burning and excessive pressure increase, burning noise, a low output and is known to have a problem of solving a narrow driving region (is limited to use only in a low speed and a low load driving region of an engine).
Thus, researches for a mixed combustion of burning diesel and gasoline at the same time to avoid the problem of the existing HCCI are carried out.
There is a method of burning a homogeneous fuel, as an example of the diesel-gasoline mixture combustion, in order to suppress knocking caused by the premixing and to control the ignition timing near the TDC by elongating the ignition delay period, by mixing gasoline with a high octane number in advance to make a homogeneous mixture within the combustion chamber during the intake stroke and by directly injecting diesel with a high cetane number for the compression ignition into the combustion chamber for the compression ignition during the compression stroke.
This diesel-gasoline mixture combustion enables to secure ignition of gasoline even under the high EGR condition in comparison to the existing HCCI and not point combustion but volumetric combustion to improve combustion efficiency and fuel ratio than those of the existing gasoline engine because self-ignited diesel acts as an ignition source for combustion of gasoline.
Moreover, the engine employing the diesel-gasoline mixture combustion reduces products of exhaust gas in comparison to the existing diesel engine and the expensive after-treatment device making price increase may be omitted.
However, since, in the diesel-gasoline mixture combustion engine, the mixture combustion is possible only under the condition where the diesel compression ignition may occur, the diesel-gasoline mixture combustion engine must be driven at a compression ratio lower than that of a diesel engine but higher than that of a gasoline engine and a compression ratio of a cylinder thereof is equal to or higher a boundary condition where the diesel compression ignition is secured.
Thus, the diesel-gasoline mixture combustion engine is disadvantageous than the existing diesel engine in view of securing the compression ignition due to the low compression ratio and the high EFG condition in the low load driving region, while possibility of knocking becomes higher than the existing gasoline engine because of the high compression ratio in the high load driving region.
Hence, in order to improve the above-mentioned problems over all driving regions, it needs to apply the diesel-gasoline mixture combustion at a compression ratio higher than that of the existing gasoline engine and lower than that of the existing diesel engine over the low/middle load engine driving regions.
Moreover, it needs a complex combustion mode system capable of utilizing the gasoline combustion to in which a compression ratio employed in a general gasoline engine is applied at the low load and the high load, and to this end concept of a variable compression ratio must be introduced.
In this case, it needs to secure the compression ignition in the region where the diesel-gasoline mixture combustion is applied and even in a region where the gasoline combustion occurs, and to prevent knocking from being generated by the gasoline combustion in the high load region.
To this end, it needs to distinguish combustion modes and to control the same, and a technology of solving the problem caused by the existing fixed compression ratio is needed.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.